Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
  • H03F3/20—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers
  • H03F3/24—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers of transmitter output stages
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
  • H03F1/32—Modifications of amplifiers to reduce non-linear distortion
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/02—Transmitters
  • H04B1/04—Circuits
  • H04B1/0475—Circuits with means for limiting noise, interference or distortion
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
  • H04L25/03828—Arrangements for spectral shaping; Arrangements for providing signals with specified spectral properties
  • H04L25/03834—Arrangements for spectral shaping; Arrangements for providing signals with specified spectral properties using pulse shaping
  • H04L25/0384—Design of pulse shapes
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/10—Frequency-modulated carrier systems, i.e. using frequency-shift keying
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/20—Modulator circuits; Transmitter circuits
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/20—Modulator circuits; Transmitter circuits
  • H04L27/2003—Modulator circuits; Transmitter circuits for continuous phase modulation
  • H04L27/2007—Modulator circuits; Transmitter circuits for continuous phase modulation in which the phase change within each symbol period is constrained
  • H04L27/2017—Modulator circuits; Transmitter circuits for continuous phase modulation in which the phase change within each symbol period is constrained in which the phase changes are non-linear, e.g. generalized and Gaussian minimum shift keying, tamed frequency modulation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
  • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
  • H04L27/36—Modulator circuits; Transmitter circuits
  • H04L27/366—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
  • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
  • H04L27/36—Modulator circuits; Transmitter circuits
  • H04L27/366—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator
  • H04L27/367—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator using predistortion
  • H04L27/368—Arrangements for compensating undesirable properties of the transmission path between the modulator and the demodulator using predistortion adaptive predistortion
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F2200/00—Indexing scheme relating to amplifiers
  • H03F2200/451—Indexing scheme relating to amplifiers the amplifier being a radio frequency amplifier

Definitions

• the present invention relates to a communication device which is provided with pre-distortion to compensate for distortion introduced by components in the device, and a method of such pre-distortion.
• the present invention relates to the compensation of distortion by components in a transmitter.
• Components in the transmitter cause an information signal to be distorted prior to transmission to differing degrees.
• Two major factors affect this distortion, namely ramp up and ramp down, and component tolerances. Whilst individually such distortions may not affect the signal to be transmitted significantly, there is a possibility that the superposition of such signals is unacceptable as it is outside the limits defined by the appropriate telecommunication standard. For example, such distortion may cause a signal in the frequency domain to extend outside the bandwidth defined by a standard.
• each device can be measured, and the devices not meeting the necessary requirements can have the offending components altered. As will be appreciated, this increases manufacturing costs, and results in a reduction in yield over a given time.
• active pre-distortion can be provided, for example on the basis of feedback from the output of a component to be compensated for. However, this increases the complexity of the transmitter and the power consumption.
• a filter may sometimes be used to compensate for distortion by certain components. However, this is not possible for cases where the distortion varies over a 2 frequency range such as in non-linear devices (for example non-linear power amplifiers).
• GMSK used in
• GSM Global System for Mobile communications
• QPSK (used in CDMA systems) is spectrally efficient but power inefficient.
• GMSK Distortion by components in the transmitter of a communication device can reduce the benefits of these schemes in practice.
• GMSK was selected to be used in GSM on the basis that it provides a constant amplitude, and is thus power efficient.
• distortion of the signal by components in a GSM transmitter will cause variation in the amplitude envelope and thus a variation in power efficiency, so negating at least to some extent the advantages for which GMSK was chosen.
• a method for defining the relationship between frequency and amplitude of a pulse function for acting on a data stream for transmission in a telecommunications system to compensate for distortion by a component of the transmitter comprising: defining desired cost parameters on the basis of the telecommunications system requirements and the distortion to be compensated for; and defining the amplitude of the pulse function over a range of frequencies in dependence on the desired cost parameters and the distortion.
• This method enables the designer to select from an expanded choice of components for use in the transmitter. For example, it may enable a designer to use a more efficient power amplifier, because the method provides pre- 3 distortion to compensate for any variation in efficiency. Also, this same distortion can be used for each transmitter designed, and thus eliminates the need for testing and modifying individual devices during manufacture. Furthermore, once the defined pulse function has been implemented in a transmitter there is no need for further pre-distortion compensation. That is, there is no active pre-distortion - this method provides a single step static pre- distortion.
• the invention is particularly advantageous when compensating for non-linear distortion.
• Non-linear distortion may result by virtue of the component itself being non-linear.
• the compensation can be targeted at the particular frequencies where maximum distortion occurs. Compensation may be for a power amplifier. This will enable a more efficient power amplifier to be used, thus reducing power consumption of a communication device. This is made possible by the method as the power amplifier can operate in its nonlinear region without having to suffer the consequences of non-linearity as this is compensated for.
• non-linear distortion can arise when determining a pulse function which is suitable for components (linear or non-linear) having different tolerance values. This is advantageous in that the same pulse function can be used in devices employing different components. It does not, for example, need to be ascertained for each type of device before it can be put on the production line.
• a suitable pulse function for compensating for non-linear distortion can be readily determined by minimising the distortion function symbolically (as opposed to numerically). Values can then replace the symbols to determine the actual amplitude values for given frequencies for a particular transmitter. 4
• the present invention may be used to compensate for one or plurality of components. This method reduces the mathematical complexity in determining the level of distortion to be compensated for by two components.
• the telecommunication system is GSM
• the first component is a reconstruction filter
• the second a non-linear power amplifier.
• the pulse function required for compensation of the reconstruction filter is determined, and then a second pulse function which would result in the reconstruction filter compensation pulse function is compensating for the power amplifier.
• Desired cost parameters may be defined on the basis of a number of telecommunication system requirements, including TDMA and CDMA.
• TDMA time division multiple access
• CDMA Code Division Multiple Access
• a pulse function may be defined according to the method of the present invention such that a conventionally shaped Gaussian pulse may be transmitted.
• the pulse function may be defined to take other cost functions into account.
• the pulse function may be defined conventionally such that a pulse of a root raised cosine shape may be transmitted, or alternatively the pulse function may be defined to take other cost functions into account.
• the defined pulse function is generated by a pulse function generator.
• a modulator which comprises means for shaping a data stream in accordance with the defined pulse function.
• the means for shaping a data stream advantageously comprises a look-up table.
• a transceiver for a communication device comprising such a modulator and a demodulator, and a communication device, comprising such a transceiver.
• a dual mode communication device operable in a first mode in a TDMA telecommunications system in which a channel is a combination of frequency and timeslot and a second mode in a CDMA telecommunications system, comprising a modulator for modulating a data stream with a carrier signal in accordance with a predetermined modulation scheme in both the first and second modes of operation and a pulse function generator for shaping a data stream in accordance with respective pulse functions responsive to the mode of operation of the radio telephone and distortion by a component of the transmitter.
• a dual mode communication device operable in a first mode when a first set of cost parameters are desired and in a second mode when a second set of cost parameters are desired
• the radiotelephone comprising: a first pulse function generator for converting a data stream in accordance with a pulse function shaped in dependence on the first set of desired cost parameters; a second pulse function generator for converting a data stream in accordance with a pulse function shaped in dependence on the second set of desired cost parameters; and means for selecting the pulse function generator in accordance with the mode of operation of the phone; wherein at least one of the pulse functions is shaped in accordance with the relationship defined by any of the above methods.
• This communication device may be operable at a first data rate in the first mode and a second data rate in the second mode.
• the first data rate supports voice applications and the second data rate supports data applications.
• a dual mode communication device operable in a first mode when a first set of cost parameters are desired and in a second mode when a second set of cost parameters are desired
• the radiotelephone comprising: a modulator for modulating a data stream with a carrier signal in accordance with a predetermined modulation scheme in both the first and second modes of operation; a first pulse function generator for shaping a data stream in accordance with a pulse function shaped in dependence on the first set of desired cost parameters and distortion by a component of the transmitter; a second pulse function generator for shaping a data stream in accordance with a pulse function shaped in dependence on the second set of desired cost parameters and distortion by a component of the transmitter; and means for selecting the pulse function generator in accordance with the mode of operation of the phone.
• a method for selecting a modulation scheme for a communication system using a predetermined transmitter comprising: defining a pulse function for a first modulation scheme in accordance with any of the aforementioned methods; defining a pulse function for a second modulation scheme for the same desired cost parameters; 7 determining the resultant cost parameters for each scheme; and selecting the modulation scheme which gives good resultant cost parameters given the desired ones.
• root raised cosine H (f) 1 I f I ⁇ oc
• H (f) -(1 - cos(2 ⁇ (f - (T + ⁇ ) oc ⁇ f ⁇ ⁇ + oc
• the pulse shape most appropriate for each modulation scheme is quite clear.
• the system designer conventionally makes a decision on which modulation scheme based on its strengths and weaknesses and selects the appropriate pulse shape based solely on that chosen modulation scheme (i.e. a Gaussian for GMSK and a root raised cosine for QPSK). He does not take distortion of the ideal pulse shape into account when determining the modulation scheme to be used.
• the single variable of the mathematical function is set to provide an acceptable balance in the defined relationship between the cost parameters.
• the pulse shaper there is no predetermined mathematical relationship for the pulse shaper.
• the shape of the pulse is defined in order to compensate for distortion by one or more transmitter components. There is freedom to select new pulse shapes that compensate for distortion so that the transmitter can transmit a signal of appropriate (potentially ideal) shape for the chosen modulation scheme, such as a Gaussian or root raised cosine.
• pulse shapes may be chosen which provide for compensation, and furthermore allow many cost parameters to be balanced against each other since the trade-off relationship between two parameters is no longer defined so restricted. This leads to a number of interesting possibilities, which will be outlined below.
• the pulse shape in MSK it is not necessary for the pulse shape in MSK to be gaussian.
• this particular pulse 9 shape optimizes performance in terms of power efficiency it is not optimal in terms of spectral efficiency.
• the balance between BER, power efficiency and bandwidth alters. The same applies to the consequences of deviations from the root raised cosine pulse shape in QPSK modulation.
• Pulses in both the MSK system and the QPSK system can be shaped to provide a desired balance between cost functions (e.g. BER, bandwidth, power efficiency, AFC) rather than being at the mercy of existing trade-offs when only the parameters of a given shape pulse such as a gaussian or a root raised cosine are modified.
• Cost functions are functions which are positive and get smaller the better a system operates.
• the present invention suggests that the pulse shape in a modulation scheme can be used to alter the cost functions (parameters) of the scheme as well as to provide predistortion.
• This can be utilised in a number of ways. It allows existing modulation schemes to be looked at afresh with new pulse shapes to obtain performances that are better than those possible at present with conventional pulse shapes.
• the modulation schemes for particular systems could be selected on a different basis. 10
• a dual mode GSM CDMA terminal could be constructed using a single modulator.
• the pulse shape would be likely to differ for each system as the cost function (desired parameters) may also differ.
• the invention also allows problems with existing systems to be ameliorated.
• the major problem with the existing GSM system is that as data users increase they will occupy an increasing fraction of available channels. This is because if data transmission rates are to match expectations, more than one channel is needed to transmit data. If the data rate requirements are met, as the fraction of data users increases, the capacity of the system decreases. If GSM is to continue to support both voice and data users, without grinding to a halt, the number of operable channels needs to increase.
• This increase in operable channels can be achieved by modifying the pulse shape in the MSK modulation scheme to reduce the relevant cost function i.e. bandwidth or bit error rate.
• a channel can be divided either in time so that more than one frame is sent in a single time slot.
• the channel could be divided in frequency to allow more than one frequency band to be sent in an existing (wider) frequency band.
• a data user could then send data at a faster rate by using 'subchannels' within a channel allocated by the system without reducing the capacity of the system. Without the subchannels the only way the data rate could be increased would be by using more than one channel to transmit data. For every additional channel utilised, there would be a corresponding reduction in the number of channels available, for both voice and data transmissions. 11
• the two types of user, fast data rate and slow data rate could co-exist in the same system with only minimal changes to the GSM infrastructure (e.g. software).
• the proposed half rate GSM could be used, in which a half rate channel (where only one slot every other frame is dedicated per subscriber) is used for slow data rate users, This arrangement would accordingly allow GSM to evolve to become useful as a high data rate system, extending its practical life.
• GSM there are 8 timeslots and 175 frequency bands. By dividing each channel into two up to twice the users could be accommodated on the existing channel. This would significantly effect the future data user capacity of the system
• pulse shaping would allow the cost functions in a QPSK modulation scheme to be optimized. This would allow the power efficiency to increase with perhaps some resulting increase in the BER and decrease in spectral efficiency.
• the freedom to select a pulse shape unconstrained by conventional root raised cosines and gaussian pulses allows the cost function in any of these modulation schemes to be optimized to meet desired parameters. This provides greater freedom in designing a third generation or enhanced second generation system, having desired parameters that are considered as set of weighted costs in an optimization calculation.
• figure 1 (a) illustrates a conventional GSM transmitter
• figure 1 (b) illustrates a conventional CDMA transmitter
• figure 1 (c) shows the GSM frame structure
• figure 2 illustrates a GSM transmitter according to an embodiment of the present invention
• figure 3 illustrates a CDMA transmitter according to an embodiment of the present invention
• figure 4(a) is a block diagram illustrating terms used in compensating for a single component
• figure 4(b) is a block diagram illustrating terms used in compensating for two components, namely an A/D converter and a reconstruction filter
• figure 4(c) illustrates a GSM transmitter according to an embodiment of the present invention
• figure 5(a) shows the characteristics of an ideal and a typical amplifier
• figures 5(b) to (d) show required, uncompensated and compensated spectrums of the typical amplifier
• figures 6(a) and (b) illustrate pulse shapes which provide predistortion for a typical power amplifier
• figures 6(c) to (f) illustrate the uncompensated , ideal and compensated spectrum of the typical power amplifier
• figure 7 is an enhanced GSM transmitter according to an embodiment of the present invention
• figure 8 is a dual mode GSM/CDMA transmitter according to an
• FIG. 1a illustrates a GSM transmitter.
• GSM conventionally comprises a frame structure as shown in Figure 1c.
• a bit sequence 101a to be transmitted is input to a frame builder 102a of the transmitter, which puts the bits in the appropriate portion of a burst within a time slot of a TDMA frame.
• the bit stream is then forwarded to a modulator 104a.
• this modulator would be a GMSK modulator, and thus the signal would be put through a Gaussian filter.
• a clock or carrier generator 105a provides the carrier signal.
• the modulated signal is input to a digital analogue converter 107a. This analogue signal is then reconstructed by reconstruction filter 108a.
• This filter might typically comprise a switch capacitor filter for performing some of the spectral shaping and an analogue filter, such as an RC filter, for mainly dealing with residual shaping.
• an analogue filter such as an RC filter
• FIG. 1(b) illustrates a conventional Code Division Multiple Access (CDMA) transmitter.
• CDMA conventionally comprises a frame made up of a dedicated physical data channel (DPDCH) and a dedicated physical control channel (DPCCH).
• DPDCH dedicated physical data channel
• DPCCH dedicated physical control channel
• a bit sequence 101 b to be transmitted is input to a frame builder 102b of the transmitter, which puts the bits in the appropriate part of the frame (i.e. in the DPDCH).
• This Gold Code Encoder 103b operates as follows.
• the output of the Gold Code Encoder 103b is a sequence with N x M terms having the following elements :
• a modulator 104b modulates these MN chips output by the Gold Code Encoder 103b on to a carrier, which is output by clock or carrier generator 105b.
• the modulator 104b is a QPSK modulator, as is generally used in CDMA systems such as IS95, and outputs a signal having a root raised cosine pulse function.
• the output of the modulator 104b is input to a digital to analogue converter 107b.
• the analogue signal is then reconstructed by a reconstruction filter 108b.
• a reconstruction filter might typically comprise a switch capacitor filter for performing some spectral shaping and an analogue filter, such as an RC filter network, for mainly dealing with residual shaping.
• the signal output by the antenna 110a, b of these systems will not be of ideal Gaussian / root raised cosine shape due to its distortion by components of the transmitter. Compensation of such distortion may be provided as outlined in the description of the background of the invention. However, as explained these solutions have disadvantages associated with them.
• a new method of predistortion is provided in which the modulator modulates the information signal with a pulse shaped to compensate for distortion by one or more of the transmitter 15 components. This will be explained further with reference to Figures 2 and 3 below.
• FIG. 2 illustrates a GSM transmitter according to an embodiment of the present invention.
• a number of components of the transmitter correspond to those of the conventional GSM transmitter shown in Figure 1(a): the bit sequence 201 , frame builder 202, modulator 204 and D/A converter 207, reconstruction filter 208, power amplifier 209 and antenna 210 correspond to their counterparts 101a, 102a, 104a, 107a-110a respectively.
• the main difference lies in the means for providing the modulator with a carrier.
• the carrier generator 105a of the conventional transmitter which provides a Gaussian signal in the frequency domain, is replaced with a clock 205 and a look up table 206 comprising data for adaptively shaping the pulse function to compensate for component distortion.
• Figure 3 illustrates a CDMA transmitter according to the present invention, in which the main difference over the transmitter of Figure 1(b) lies in the means for providing the modulator with a carrier.
• the carrier generator 105b of the conventional CDMA transmitter which provides a root raised cosine signal in the frequency domain, is replaced with a clock 305 and a look up table 306 comprising data for adaptively shaping the pulse function to compensate for component distortion.
• the look-up table comprises data defining the amplitude of the pulse function over a range of frequencies which meet the telecommunications system requirements and compensate for component distortion.
• Figure 4(a) is a block diagram illustrating the terms used in an initial equation which compensates for a single component.
• F(t) is the symbol reflecting the function of the component (e.g. reconstruction filter) to be compensated for
• C(t) is symbol indicative of the desired pulse to be output (for example a Gaussian pulse in GSM or a root raised cosine pulse in CDMA)
• C'(t) is the symbol reflecting the pulse required to compensate for distortion by the component.
• the distortion function for such a component is defined by:
• This distortion (cost) function can be solved using any standard equation solving technique. That is, the distortion cost function is minimised. Numeric values can then replace the symbols to determine the actual value of C'(t) for a particular type of transmitter.
• Figure 4(b) is a block diagram illustrating the terms used in an initial equation which compensates for a plurality (in this case two) components.
• F vD(t) is the symbol reflecting the function of an A/D converter filter to be compensated for
• FREF(t) is the symbol indicative of the function of a reconstruction filter to be compensated for
• C(t) is the symbol for desired pulse to be output (for example a Gaussian pulse in GSM or a root raised cosine pulse in CDMA)
• C'(t) is the symbol for the pulse required to compensate for distortion by the components.
• the distortion function for the components is defined by: 17
• this distortion (cost) function can be determined using any standard equation solving technique. That is, the distortion function is minimised. Numeric values can then replace the symbols to determine the actual value of C'(t) to be stored in the look-up table of a particular transmitter.
• the components may be treated as a composite, or they may be compensated for individually.
• the latter technique is useful, for example, to reduce the mathematical complexity in solving the equations, and is exemplified with respect to the GSM transmitter of Figure 2, as shown in Figure 4(c). In this case, predistortion of the power amplifier 409 and the reconstruction filter is required.
• the power amplifier is treated as having been removed from the path between the modulator 404 and the antenna 410, to a position between the look up table 406 and the carrier input of the modulator 404, as indicated in dotted lines in Figure 4(c).
• the predistortion to compensate for errors caused by the reconstruction filter 408 is determined by solving equation 1 above, where F(t) is the function of the reconstruction filter 408.
• a cost function is determined which will provide the optimum predistortion over a range of tolerance values. This is applicable to both linear and non-linear devices, and enables a single look-up table to be used for a variation of components. Hence, it avoids the 18 need to measure individual devices during manufacture.
• the cost function may be defined by:
• F. is the function of the component having a first tolerance value to be compensated for; where F 2 is the function of the component having a second tolerance value to be compensated for; where F is the function of the component having a third tolerance value to be compensated for; C(t) is the desired pulse to be output;
• Cj(t) is the pulse required to compensate for distortion by the component having the first tolerance value
• C 2 (t) is the pulse required to compensate for distortion by the component having the second tolerance value
• C ) is the pulse required to compensate for distortion by the component having the third tolerance value
• ⁇ , ⁇ and ⁇ are weighting factors.
• Typical tolerance values for the components might be: First value +5%
• weighting factors can be varied so as to tailor the results. Values for the cost function are then determined which minimise the cost function and thus give a pulse shape which compensates for distortion over the range of tolerance values (e.g. +5% +0 -5%).
• Figure 5(a) shows the characteristics of a typical power amplifier against the ideal linear amplifier 53. At low amplitudes, the power amplifier is linear as indicated by the part of the line 52, and there is thus a linear relationship between amplitude and output power. Ideally, this linear relationship would be maintained at higher amplitudes as indicated by the line 53.
• power amplifiers have a non linear region at higher amplitudes, as indicated by the curved part 51. Variations occur primarily due to the use of inexact component values and during ramp up and ramp down of the burst. It is more power efficient to operate power amplifiers in this non linear region.
• the power amplifier is chosen to operate in its more power efficient, yet non-linear region.
• the non-linearity of this power amplifier is around 5%, and predistortion is determined as follows, so that it can be input in the look up table 206 of Figure 2 to counteract this non-linearity.
• Figure 5(b) compares the spectrum of a non linear amplifier 54 with a linear one 55.
• Figure 5(c) shows the result of using pulses that have been derived to compensate for the non linearity using a conventional optimisation procedure, for example as described with reference to Figure 4 above.
• the predistorted (compensated) power amplifier has a narrower spectrum than the non compensated power amplifier. Indeed, it even overcompensates at the regions where variations in amplitude are most rife during ramp up and ramp down, namely at around 200kHz and, more critically, at around 400kHz.
• Figure 5 (d) illustrates the spectrum including distortion from the reconstruction filter (this component is likely to cause a substantial amount of the overall transmitter component distortion), and also shows the GSM mask, 57.
• distortion from other transmitter components may cause the 21 spectrum of the uncompensated non-linear amplifier to shift outside the GSM mask 57, and thus preventing the transmitter from being allowed for use in the GSM system.
• the power amplifier compensation of this embodiment is sufficient to ensure that the total distortion by transmitter components does not cause this to happen since there is a clear margin between the spectrum 56 (which already includes the reconstruction filter distortion), and the mask 57.
• compensation could alternatively be provided for a number of transmitter components. This could either be done at the same time as the power amplifier compensation, before or afterwards.
• Gaussian and root raised cosine pulses have been considered for use in modulators of telecommunications systems to date.
• Laurent has suggested that a Gaussian pulse can be approximated by the superposition of AM pulses (C 0 C, ...etc.), these pulses being a fixed family of pulses which are functions of cos and sin.
• a totally different approach has been taken, as is outlined below.
• the fixed function components in Laurent's superposition expansion are replaced by one or more functions representing respective unknown pulse components.
• cost functions are looked at (e.g. component distortion 22 such as non linearity, BER, bandwidth, amplitude, AFC). That is, the errors from the values that the particular system requires and the amount of predistortion to be compensated for are considered.
• the weightings of the cost functions can be varied so as to tailor the results. Values for each function are then determined, for example using an optimiser, which minimise these cost functions and thus give a pulse shape which meets the specified system requirements.
• the steps required to obtain an appropriate frequency spectrum taking into account power amplifier distortions are as follows: 1. Predistort two Laurent pulses, CO and C1 to compensate for reconstruction filter tolerances and A/D specifications;
• Figures 6(a) and (b) show the results of the optimisation, Figure 6(a) showing the requisite first distorted Laurent pulse CO and Figure 6(b) showing the second, C1.
• Figure 6(c) illustrates the spectrum of an uncompensated power amplifier having a 5% non linearity
• Figure 6(d) illustrates the desired ideal spectrum of the output signal (i.e. that of an ideal linear power amplifier).
• Figure 6(e) shows the spectrum of a power amplifier compensated in accordance with the unknowns determined by the above process
• Figure 6(f) shows the three spectrums illustrated in Figures 6(c) to (e) together for comparison. 23
• the replacement of fixed function components in Laurent's superposition expansion by one or more functions representing respective unknown pulse components can be extended beyond determining appropriate pulse shapes for providing predistortion.
• appropriate pulse shapes are determined which, as well as providing for predistortion, also provide improved cost functions than would be obtained using conventional shaped pulses in telecommunications system (e.g. better than would be obtained using a Gaussian pulse shape for GSM or root raised cosine for CDMA).
• the pulse shaping can be determined in a single optimising step, or in consecutive steps. Firstly, a first pulse shape which meets the cost function requirements other than distortion error could be determined, for example as will be explained below, followed by determination of the pulse shape which would result in that first pulse shape taking into account component distortion.
• the method of dsetermining the appropriate pulse shape can be implemented as follows :
• PULSE J K which is as yet unknown, but for which we wish to determine an appropriate value depending upon requisite error function requirements.
• PULSE is unknown as yet, but is, in this embodiment it is read, non zero and of maximum length 8.
• equation (3) can be identified as to whether it is real or imaginary (assuming that the pulse function is real).
• N-V N-l eV ⁇ n ⁇ * real -
• equation 3 e.g. for a simple receiver
• bit 0 C y _4 is transmitted at time (N + 4)7 " as it is on its own. It is imaginary, and the interfering (ie other imaginary) pulses must be taken into account.
• the real terms in this expression can be totally ignored both for the interfering terms and the absolute value of the pulses.
• the interference should be minimised.
• the BER performance can, for example, be improved by making the terms Pulse [0] at (N + 4)r large compared to the absolute value of all the other terms.
• the absolute value of the pulse at time AT can be calculated in terms of the unknown pulses.
• the pulse is required to meet certain criteria with regard to power, BER, AFC and bandwidth. Hence, error functions for these are determined.
• the oversampling rate can be altered depending upon the level of pulse sampling required.
• the amplitude and BER costs are calculated for AT taking each of the above values.
• the total cost for each is the addition of all the 8 expressions obtained over the possible sequences.
• the derivative of the pulse functions (which at this stage are still unknown) are required. This derivative can be approximated as being the difference between two adjacent pulse values.
• the bandwidth for a pulse is given by :
• Pulse [0][t] is non zero for 0 ⁇ t ⁇ 9T Pulse [l][t] is non zero for 0 ⁇ t ⁇ IT
• the unknown pulses are : rp
• Pulse [0][2r+— ] X ⁇ ' ⁇ ,19
• Pulse[l][2r+ ⁇ ] Xn9 o '
• Adjacent sample points have adjacent numbers and the set of unknowns becomes : 29
• the bandwidth for the second component is to be determined, then a similar expression needs to be determined for PULSE [1]. This is as follows :
• the pulse can be specifically designed based on system requirements by weighting the above error functions (for example 0.3 for power, 0.3 for BER and 0.4 for bandwidth or if a system requires only, for example, bandwidth considerations, 0 for power and BER and 1 for bandwidth). More weight can be added to whatever is causing a problem. The only restriction is that the total weighting must equal + 1.
• the carrier generator 105a, 105b of the conventional transmitter which provides a Gaussian signal (for GSM) / root raised cosine signal (for CDMA) in the frequency domain, is replaced with a clock 205 and a look up table 206 comprising data for adaptively shaping the pulse function to compensate for component distortion.
• the data of the look up table also adaptively shapes the pulse function to meet other cost function requirements. This enables the modulator type to differ from the type conventionally used by a particular telecommunication system.
• the modulator may be a GMSK modulator, as is conventional.
• the modulator 304 may be a QPSK modulator as is generally used in CDMA systems such as IS95. However, alternatively, it may be a continuous phase modulator such as that used in MSK modulation.
• the bandwidth of the signal output by the modulator 204, 304 is related to the spectrum of the pulses that are used to make up the lookup table 206, 306.
• EDGE For the further evolution of GSM (EDGE), it has been proposed to improve the data rate by 3 by changing the modulation scheme.
• the present invention enables the data rate to be increased by designing a pulse which meets the appropriate cost functions. Consequently, the existing infra-structure can be used with its channels divided either in frequency or time. 31
• FIG. 7 shows an enhanced GSM transmitter which could be used in such a system.
• This transmitter is similar to present GSM transmitters, and in this embodiment the modulator 204 is an MSK modulator.
• this enhanced GSM transmitter comprises 2 lookup tables 206a and 206b, which define different pulse functions for acting on the bit sequence 201.
• Lookup table 206a comprises data defining a pulse function which can act on speech with the current data rate of 9.6 kilobits per second.
• the data of this lookup table could correspond to the Gaussian curve, so that it provides the same modulation (ie GMSK) as is currently used in GSM.
• it comprises data defining a pulse function according to the present invention, for example one which is optimised for a more stringent cost function and provides predistortion.
• Lookup table 206b comprises data which defines a pulse function for acting on the bit sequence 201 for data applications.
• the pulse function is designed based on different cost functions from the ones required for the voice applications, as clearly data applications require a much improved data rate.
• Switch 211 provides connection between the modulator and the appropriate lookup table for the application required.
• Figure 8 shows a dual mode GSM/CDMA transmitter.
• This transmitter is provided with a common modulator 604.
• the transmitter having two lookup tables 606a and 606b, which, in this embodiment respectively define pulse shapes which meet the cost function requirements of GSM and CDMA and provide compensation for transmitter component distortion.
• a number of components can be used for both GSM and CDMA operations and where two components are required a switch is included, the switch between them depending on the operation of the transmitter. For example, if in CDMA mode the bit sequence 601 would need to be encoded by a gold code encoder 603.
• switch 611 provides a connection so a bit sequence can be shaped according to the data in this lookup table.
• switch 612 is provided so that the power amplifier is connected to the filter network for the appropriate mode of operation of the transmitter.
• Figure 9 shows a typical receiver.
• a received signal is put through at least 1 IF stage 501 to reduce its frequency to a base band frequency and then the signal is split into its I and Q components and the carrier is removed from the signal, using mixers 502a and 502b and low pass filters 503a and 503b.
• the signal is then converted from an analogue signal into a digital signal by the A/D converters 504a and 504b and forwarded to the Demodulator stage 505.
• demodulation, any equalisation, and decoding etc is performed.
• Figure 10(a) and (b) show how a pulse shape can be determined according to the present invention which compensates for component distortion without increasing the phase error.
• the first modulation table provides a pulse function which compensates for distortion by a linear component (reconstruction filter) and the second modulation table provides a pulse function which also compensates for distortion by a power amplifier.
• These 33 pulse shapes could be stored in a look up table of a phone such as the Nokia 5100 and 6100 series (e.g. 5110, 5130, 6130, 6150).
• the present invention includes any novel feature or combination of features disclosed herein either explicitly or any generalisation thereof irrespective of whether or not it relates to the claimed invention or mitigates any or all of the problems addressed.
• Annex 1 which is on the file of this application is hereby incorporated by reference.

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• Engineering & Computer Science (AREA)
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• Signal Processing (AREA)
• Power Engineering (AREA)
• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Spectroscopy & Molecular Physics (AREA)
• Transmitters (AREA)
• Amplifiers (AREA)
• Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
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• Mobile Radio Communication Systems (AREA)
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• 1999
  • 1999-01-21 GB GB0018666A patent/GB2349317A/en not_active Withdrawn
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